Magnetically aligning multilayer graphene to enhance thermal conductivity of silicone rubber composites

The increasing demand for packaging materials calls for new technologies to achieve excellent thermal conductivity of polymer composites with low content of thermal conductive filler. This article prepared a kind of magnetically functionalized multilayer graphene (Fe₃O₄@MG) via electrostatic interactions, which efficiently enhanced the thermal conductivity of silicone rubber (SR) composites by the alignment of Fe₃O₄@MG in an external magnetic field. The morphology and structure of the Fe₃O₄@MG together with the thermal conductivity of corresponding Fe₃O₄@MG/SR composites were systematically investigated by SEM, TEM, XRD, elemental mapping, and thermal conductivity tester. The obtained results showed that Fe₃O₄@MG was induced to form chain-like bundles in silicone rubber matrix under the applied magnetic field, which enhanced the MG–MG interaction, and formed effective thermal pathways in the alignment direction. Furthermore, as coating mass ratio of Fe₃O₄@MG increased, the thermal conductivity of randomly oriented Fe₃O₄@MG/silicone rubber composites (R-Fe₃O₄@MG/SR) decreased gradually, whereas the through-plane thermal conductivity of vertically aligned Fe₃O₄@MG/silicone rubber composites (V-Fe₃O₄@MG/SR) increased even filled with same contents of thermal conductive filler. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47951.     

Layer-by-layer assembly of graphene/polyaniline multilayer films and their application for electrochromic devices

Graphene/polyaniline (PANI) multilayer films were prepared via alternate deposition of negatively charged graphene oxide (GO) and positively charged PANI upon electrostatic interaction, followed by the reduction of their GO components with hydroiodic acid. The thickness of the multilayer film increased linearly with the number of its bilayers and that of each bilayer was measured to be about 3 nm. Cyclic voltammetry studies indicated that these thin composite films were electroactive, and their redox reactions were related to the insertion-extraction of counter ions in PANI layers. Furthermore, the composite films were tested to be promising electrode materials for electrochromic devices even without using the conventional indium tin oxide (ITO) electrodes.     

Thermal release of hydrogen retained in multilayer graphene films prepared by mist-chemical vapor deposition

In this study, we investigated the absorption and thermal desorption processes of H and H₂O and the thickness of multilayer graphene films deposited on Cu foils using a mist-chemical vapor deposition method. Ion beam analysis techniques such as nuclear reaction analysis (NRA), elastic recoil detection (ERD), and Rutherford backscattering spectrometry (RBS) were employed. The RBS measurements revealed that the thickness of the multilayer graphene films was approximately 8 ± 3 nm (24 ± 9 layers). The depth distribution of H was analyzed using NRA and ERD. Based on these measurements, the residual H/C ratio for multilayer graphene was estimated to be approximately 0.03 in the bulk and 0.88 on the top-most surface. Additionally, the thermal desorption temperature for H from the multilayer graphene film was less than 373 K, which was much lower than that from isotropic graphite bulk (approximately 673 K). These results suggest that the thermal release of H did not occur because of desorption from sp²- and sp³-hybridized C atoms, such as intercalation and defect sites. Instead, it occurred owing to the desorption of H₂O adsorbed near the surface.     

Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation

We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O₂ gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics.     

Correlating and predicting polymer thermal conductivities

General correlations were developed to estimte polymer thermal conductivities for four cases—amorphous polymers above the glass temperature; amorphous polymers below the glass temperature; semicrystalline polymers above the melting temperature, and semicrystalline polymers below the melting temperature. The correlations based on readily available parameters (mer weights, densities at 25°C, specific heats) yielded calculated thermal conductivities that deviated by only 1%–2% from experimental values.     

Electromagnetic loss mechanisms in antimony doped tin oxide and reduced graphene oxide multilayer films

In this paper, the antimony doped tin oxide (ATO) and reduced graphene oxide (rGO) were prepared by coprecipitation method and modified Hummers’ method, respectively. Both were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and vector network analyzer (VNA). The as-prepared rGO showed typical sheets-like structure and the as-prepared ATO showed typical nano-particle shape. Then, the ATO and rGO multilayer films were designed and the microwave absorption performances were numerically evaluated using finite element methods. The results showed that synergetic effects of interfacial polarization resonance and 1/4λ elimination were stimulated in 10-layerd 1.8 mm multilayer films to give a minimal reflection loss of −45.2 dB @ 16.1 GHz, and −10 dB bandwidth of 5.4 GHz, which was enhanced more than −30 dB in ATO or rGO monophase absorbers with the same thickness. This work provides a novel technical route to realize high-performance microwave absorbers in terms of simultaneously stronger absorption, broader absorption bandwidth and smaller thickness to facilitate higher flexibility and stability for practical applications.     

Estimation on thermal conductivities of filled polymers

A new thermal conduction model is proposed for filled polymer with particles, and predicted values by the new model are compared with experimental data. The model is fundamentally based on a generalization of parallel and series conduction models of composite, and further modified in taking into account that a random dispersion system is isotropic in thermal conduction. The following equation is derived from the new model; log λ = V · C₂ · log λ₂ + (1 ? V) · log(C₁ · λ₁). Therefore, when thermal conductivities of polymer and particles (λ₁, λ₂) are known, thermal conductivity of the filled polymer (λ) can be estimated by the equation, with any volume content of particles (V). The new model was proved by experimental data for filled polyethylene, polystyrene and polyamide with graphite, copper, or Al₂O₃.     

Highly thermal conductive graphene/carbon nanotubes films with controllable thickness as thermal management materials

As electronic devices continue to be integrated and miniaturized, the increased system power density leads to a continued increase in operating temperature, ultimately leading to degradation of stability and performance. Therefore, the development of thermal management materials with superior thermal conductivity is urgently needed. Herein, full-carbon graphitized graphene/carbon nanotubes (CNTs) (gGC) films with controlled thickness were fabricated through compositing followed by compaction. The appropriate amount of CNTs doping enlarges the crystallinity and improves the stacking order of the composite film during the structural evolution process. Ultimately, the thermal synergy between CNTs and graphene sheets accelerates the propagation of phonons and endows the free-standing gGC film with good performance. The flexible gGC film shows an in-plane thermal conductivity of 1280.3 W/mK, an electrical conductivity of 6559 S/cm, and the foldability of 10,000 times at a thickness of 30 μm. Moreover, gGC films with controllable thicknesses were successfully prepared through a convenient “multilayer compaction” strategy, which allows thick gGC films to maintain a high level of thermal dissipation, while effectively mitigating the rapid decline in thermal conductivity of the films with increasing thickness. This work provides a general method for the realization of large-scale and convenient production of high-thermally conductive thick films.     

Quantum-squeezing effects of strained multilayer graphene NEMS

Quantum squeezing can improve the ultimate measurement precision by squeezing one desired fluctuation of the two physical quantities in Heisenberg relation. We propose a scheme to obtain squeezed states through graphene nanoelectromechanical system (NEMS) taking advantage of their thin thickness in principle. Two key criteria of achieving squeezing states, zero-point displacement uncertainty and squeezing factor of strained multilayer graphene NEMS, are studied. Our research promotes the measured precision limit of graphene-based nano-transducers by reducing quantum noises through squeezed states.     

Low-temperature synthesis of multilayer graphene/amorphous carbon hybrid films and their potential application in solar cells

ABSTRACT: The effect of reaction temperature on the synthesis of graphitic thin film on nickel substrate was investigated in the range of 400 [DEGREE SIGN]C to 1,000 [DEGREE SIGN]C. Amorphous carbon (a-C) film was obtained at 400 [DEGREE SIGN]C on nickel foils by chemical vapor deposition; hybrid films of multilayer graphene (MLG) and a-C were synthesized at a temperature of 600 [DEGREE SIGN]C, while MLG was obtained at temperatures in excess of 800 [DEGREE SIGN]C. Schottky-junction solar cell devices prepared using films produced at 400 [DEGREE SIGN]C, 600 [DEGREE SIGN]C, 800 [DEGREE SIGN]C, and 1,000 [DEGREE SIGN]C coupled with n-type Si demonstrate power conversion efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. A HNO3 treatment has further improved the efficiencies of the corresponding devices to 0.004%, 1.080%, 0.800%, and 0.820%, respectively. These films are promising materials for application in low-cost and simple carbon-based solar cells.     

Estimation of the effective thermal conductivities of two-phase media

A nomogram scaled in logarithmic units, and a chart are provided for estimation of the effective thermal conductivities of two-phase media. The former is useful for precise prediction, and the latter as an approximate guide to probable limiting values. The basis is a logarithmic equation proposed by Woodside & Mesmer, biased empirically following the study of about 150 experimental results for two-phase media. The practical uses are numerous, e.g. estimations of the conductivities of filled plastics, filled rubbers; metal, ceramic and other powders in fluids or gases; ocean sediment; waterlogged and frozen soil, and other materials.     

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACT

Effect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.     

Tunable bands in biased multilayer epitaxial graphene

We have studied the electronic characteristics of multilayer epitaxial graphene under a perpendicularly applied electric bias. Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electronic density-of-states in valence bands near the Fermi level. Evolution of the electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using first-principles density-functional theory including interlayer van der Waals interactions. The experimental and theoretical results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed.     

Raman spectroscopy at the edges of multilayer graphene

Individual graphene layers in a multilayer graphene sample contribute their own edges. The edge of a graphene layer laid on an n layer graphene (nLG) is a building block for the edges of multilayer graphenes. We found that the D band observed from the edge of the top graphene layer laid on the nLG exhibits an identical line shape to that of disordered (n + 1)LG. Based on the spectral features of the D and 2D bands, we identified two types of alignment configurations at the edges of bilayer and trilayer graphenes, whose edges are well-aligned from their optical images.     

Synthesis of high-quality graphene films on nickel foils by rapid thermal chemical vapor deposition

We report the synthesis of high-quality graphene films on Ni foils using a cold-wall reactor by rapid thermal chemical vapor deposition (CVD). The graphene films were produced by shortening the growth time to 10 s, suggesting that a direct growth mechanism may play a larger role rather than a precipitation mechanism. A lower H₂ flow rate is favorable for the growth of high-quality graphene films. The graphene film prepared without the presence of H₂ has a sheet resistance as low as ～367 ohm/sq coupled with 97.3% optical transmittance at 550 nm wavelength, which is much better than for those grown by hot-wall CVD systems. These data suggest that the structural and electrical characteristics of these graphene films are comparable to those prepared by CVD on Cu.     

Microstructures and thermal conductivities of reaction sintered SiC ceramics

Abstract

Reaction sintered SiC ceramics were prepared by the silicon melt infiltration method over temperatures of 1450?1550°C. The effects of the carbon and silicon contents of the starting materials as well as the sintering temperature and time on the thermal conductivities and microstructures of the ceramic materials were studied. The thermal conductivities and microstructures of the samples were characterised using thermal conductivity measurements, X-ray diffraction analysis, scanning electron microscopy, energy-dispersive X-ray spectroscopy and mercury injection porosimetry. The results showed that sintering temperature and time as well as the carbon and silicon contents of the green specimens are the main factors affecting the microstructure and porosity of reaction bonded SiC ceramics. Increasing the reaction temperature and time decreased the porosity of the ceramics. This was due to the infiltration of the silicon melt into the ceramic specimens. The thermal conductivity and porosity of the sample sintered at 1550°C for 3 h in an argon atmosphere were 102·5 W m K^?1 and 0·3% respectively.     

Physical and thermal characterization of graphene oxide modified gelatin‐based thin films

The incorporation of graphene oxide (GO) nanosheets into fish gelatin‐based films and the resulting effect on the structural, mechanical, and thermal properties of the film was investigated. Gelatin films were prepared using a casting method with d ‐sorbitol as the plasticizer and carrageenan as the stabilizer. GO nanofillers were dispersed into the gelatin network in different weight percentages. FTIR spectroscopy was used to characterize the molecular structure of gelatin films and revealed that there were no major changes in the functional groups of fish gelatin films after addition of GO and carrageenan. Tensile, differential scanning calorimetery (DSC), and thermo gravimetric analysis (TGA) tests were performed to study the effectiveness of GO nanofillers and the effect of sorbitol and carrageenan on the film properties. Addition of 1 wt% of GO significantly increased the tensile strength and the elongation modulus of the gelatin films (to 44 and 1320 MPa, respectively). This was attributed to the strong interfacial interactions between GO sheets and gelatine molecules. However, increasing the GO percentage to 2 wt% resulted in a considerable decrease in both properties (to 11 and 224 MPa, respectively). This might be due to the agglomeration of GO sheets in the gelatin network. DSC results showed a substantial increase in the T_m of the films as a result of adding carrageenan and GO to the gelatin network. This might be due to the strong combination of helical structure of carrageenan and oxygen containing groups of GO. POLYM. COMPOS., 35:2043–2049, 2014. © 2014 Society of Plastics Engineers     

High thermal conductivity and increased thickness graphene nanosheet films prepared through metal ion-free route

With the miniaturization and integration of electronic devices heat conduction becomes a serious problem. Graphene films catch research's attention because of its excellent thermal performance and graphene oxide (GO) has been used as the most common precursor to prepare graphene films. But mostly film fabricated from GO is thinner than 30 μm and much thicker films are required to meet certain requirements. Also taking GO as raw material has many disadvantages such as the introduction of massive concentrated sulfuric acid and metal ions, huge weight loss in heat treatment and so on. Herein, we propose a new strategy to prepare graphene nanosheet films (GNFs) with a thickness of 100 μm by vacuum filtration of expand graphite through weak oxidation (WEG). Unlike common strategy, WEG without any metal ions introduced instead of GO is chosen as our raw material. The addition of nonionic surfactant and the employment of microfluidization can stabilize WEG dispersion. After graphitization at 2800 °C WEGF is transferred to GNF. The obtained 100 μm-thick film possesses a decent in-plane thermal conductivity (TC) of 760 W/mk and electrical conductivity (EC) of 5.2×10⁵ S/m. Thick films with high TC can guarantee passing more heat flux and fill in larger gaps inside devices.